A meme involving giant, disfigured Touhou heads. It originated on 2ch in early 2008 when an Anon tried to make a Shift_JIS portrait of Reimu and Marisa and failed horribly. Another Anon thought the failed attempt was humorous and drew the infamous original picture (shown below right). Ever since that fateful day, yukkuri heads have dominated parts of NicoNico and made cameos in numerous other pieces of Touhou fanart. Their catchphrase “Yukkuri shiteitte ne!” literally means “Do it slowly!”, but has been more loosely translated as “Take it easy!” by the English-speaking Touhou community. This remains the basis for many of the jokes centered on yukkuris.

Since then, writers and artists in Japan have produced a huge amount of yukkuri material, making an entire ecosystem based around their various forms. Even ZUN appears to have made a small official acknowledgement of the yukkuri phenomenon, with a line of narration reading “The rocket has been destroyed!! This is no time to be taking it easy!!” in Silent Sinner in Blue Chapter 12.
From what has been gathered through comic shorts and translations, mostly from anonymous contributors on 4chan’s /jp/ image board, this is a peek at the yukkuri world.